Definitely commutators. There is no good guide that explains them well, so I kinda had to look at reconstructions and algorithm sheets to finally know how to perform them intuitively.
Cycles was the hardest thing for me in the beginning. But now I don't find them a problem at all... in fact I almost like it when there are lots of cycles, i see it as a challenge
At the end of the day once you memo the cycles... you dont have to worry about the fact that they are there. Just follow your memo.
At first it was parity. I didn't understand it from the tutorial I watched. So I taught myself my own after a while. Later found out that Zane went on to use the exact same method in his video.